The catalytic epoxidation of olefins using a silver-based catalyst has been known for a long time. Conventional silver-based catalysts have provided the olefin oxides notoriously in a low selectivity. For example, when using conventional catalysts in the epoxidation of ethylene, the selectivity towards ethylene oxide, expressed as a fraction of the ethylene converted, does not reach values above the 6/7 or 85.7 mole-% limit. Therefore, this limit has long been considered to be the theoretically maximal selectivity of this reaction, based on the stoichiometry of the reaction equation7C2H4+6O2=>6C2H4O+2CO2+2H2O,cf. Kirk-Othmer's Encyclopedia of Chemical Technology, 3rd ed., Vol. 9, 1980, p. 445.
Modern silver-based catalysts however are highly selective towards olefin oxide production. When using the modern catalysts in the epoxidation of ethylene the selectivity towards ethylene oxide can reach values above the 6/7 or 85.7 mole-% limit referred to, for example 88 mole-%, or 89 mole-%, or above. Such highly selective catalysts, which may comprise as their active components silver, rhenium, at least one further metal and optionally a rhenium co-promoter, are disclosed in U.S. Pat. No. 4,761,394, U.S. Pat. No. 4,766,105, EP-A-266015 and in several subsequent patent publications.
The silver based catalysts are subject to an aging-related performance decline during normal operation and they need to be exchanged periodically. The aging manifests itself by a reduction in the activity of the catalyst. Usually, when a reduction in activity of the catalyst is manifest, the reaction temperature is increased in order to compensate for the reduction in activity. The reaction temperature may be increased until it becomes undesirably high, at which point in time the catalyst is deemed to be at the end of its lifetime and would need to be exchanged.
A reaction modifier, for example an organic halide, may be added to the feed to an epoxidation reactor for increasing the selectivity (cf. for example EP-A-352850). The reaction modifier suppresses the undesirable oxidation of olefin or olefin oxide to carbon dioxide and water, relative to the desired formation of olefin oxide, by a so-far unexplained mechanism.
The optimal quantity of the reaction modifier depends on the epoxidation reaction conditions and on the type of catalyst used. Conventional catalysts have relatively flat selectivity curves for the modifier, i.e. the curves of the selectivity as a function of the quantity of the reaction modifier show that the selectivities are almost invariant over a wide range of reaction modifier quantities, and this property does virtually not change as a function of the reaction temperature and during prolonged operation of the catalyst. Therefore, when using a conventional catalyst, for optimum selectivity the quantity of the reaction modifier can be chosen rather freely and it can remain substantially the same during the entire lifetime of the catalyst.
By contrast, the highly selective catalysts tend to exhibit relatively steep selectivity curves for the modifier, viz. for the highly selective catalysts the selectivity varies considerably with relatively small changes in the quantity of the reaction modifier, and the selectivity exhibits a pronounced maximum, i.e. an optimum, at a certain quantity of the reaction modifier. This has been illustrated in EP-A-352850 (cf. FIG. 3 therein). Moreover, the selectivity curves and more in particular this quantity of the reaction modifier where the selectivity is at optimum tend to change with the reaction temperature and, thus, during the catalyst life.
Consequently, when employing the highly selective catalysts in combination with a reaction modifier, the selectivity may vary to an undesirably large extent with changes of the reaction temperature and over the lifetime of the catalyst. Namely, when the reaction temperature is changed, for example to compensate for a reduction in the activity of the catalyst, it represents itself as a problem to maintain reaction conditions which are optimal with respect to the selectivity towards the olefin oxide production.